Does nothing... how to troubleshoot?

Started working after a while. Don't know why.

We had a similar issue with this module "just not working" with no error message or anything else.

The issue for us was that is the number is invalid, ie. the area code doesn't exist, then the formatting doesn't happen, and no error message is output anywhere. This makes it very confusing. We sanitize our database for dev work, and our sanitization process just randomizes the phone numbers, which led to invalid phone numbers.

I don't know if this was the same issue the OP had.

If the number isn't valid then an exception is thrown and then caught here in this funcion:

  public function viewElements(FieldItemListInterface $items, $langcode) {
    $element = [];
    foreach ($items as $delta => $item) {
      try {
        if ($this->getSetting('link')) {
          $element[$delta] = $this->viewLinkValue($item);
        }
        else {
          $element[$delta] = $this->viewFormattedValue($item);
        }
      }
      catch (\Exception $e) {
        $element[$delta] = $this->viewPlainValue($item);
      }
    }

    return $element;
  }

A watchdog message should be logged if the exception is thrown.

Here's the patch.